In semiconductor fabrication, it is often desirable to anisotropically etch thick dielectric films with high aspect ratio features (e.g., 4:1 or greater) without excessive erosion of an accompanying photoresist. The application of a low pressure, high ion flux process (e.g., a high density plasma source) is advantageous to avoid aspect ratio dependent etching (ARDE) and across chip line variation (ACLV) effects. High density plasmas, such as from an inductively coupled source, operate at lower pressures due to electron acceleration normal to chamber boundaries and provide for increased ion density and flux.
By exciting the plasma with a solenoidal coil structure, the electron mean free path can be made larger than the chamber dimensions and hence the operating pressure can be lower and the fractional ionization higher than in traditional capacitive (or parallel plate) etch chambers. Both of these properties yield reduced ARDE and ACLV.
These advantages have been demonstrated for high density plasma etching of silicon oxide for feature sizes as small as 0.175 .mu.m. Many levels used in modern semiconductor devices, however, consist of both silicon oxide and silicon nitride layers, or a silicon nitride layer alone. In hardmask applications, a thick nitride layer exists, either alone or with a thick oxide layer, and cannot be etched in a high density plasma using present state-of-the-art technology due to poor photoresist selectivity and subsequent loss of image integrity. The high dissociation level in high density plasma reactors often leads to isotropic etching by fluorine atoms in fluorocarbon gases.
To avoid these drawbacks, capacitively coupled tools are used to etch thick nitride or nitride/oxide layers such as hardmask. These tools suffer from severe reactive ion etching (RIE) lag as the feature size is decreased, and raw process time (RPT) increases substantially. In addition, selectivity to photoresist is compromised when a reduced polymer concentration is used in these tools, while increasing the polymer concentration results in premature etch stop as the feature sizes are further decreased.
Nonetheless, there are several advantages in using high density plasma. For example, the ion energy is highly tunable in high density plasma because the wafer bias is independently controlled, thereby providing improved control of resist loss. For this reason, a selective nitride etch of high aspect ratio features in high density plasma is desirable. To date, selective high density plasma etching for high aspect ratio silicon nitride levels has remained elusive.
Current state-of-the-art for high density plasma nitride etching in high aspect ratio holes includes mixtures of either CH.sub.3 F/CO (SRDC) or C.sub.2 F.sub.6 /O.sub.2 (BTV). Neither of these systems provides enough selectivity to photoresist for etching of nitride layers greater than 500 .ANG. thick. At aspect ratios greater than 2:1 (0.35 .mu.m groundrules), the CH.sub.3 F/CO chemistry suffers from etch stop. C.sub.2 F.sub.6 /O.sub.2 chemistry has less than 1:1 selectivity even for shallow features.
FIG. 1 illustrates the use of an oxygen free polymerizing process (C.sub.2 F.sub.6 /CH.sub.3 F), which exhibits nitride etch rates of 3500 .ANG. min.sup.-1 on blanket nitride layer 10 through an oxide or photoresist 20. Blanket nitride layer 10 is formed over a silicon substrate 15. The etch profile is isotropic, however, and an undesirable undercut 30 exists at the nitride/oxide or nitride/photoresist interface 35.
Although the art of etching silicon nitride is well developed, some problems inherent in this technology still exist. One particular problem is etching for high aspect ratio silicon nitride levels. Therefore, a need exists for a process for etching silicon nitride with a high aspect ratio while maintaining photoresist selectivity and avoiding subsequent loss of image integrity.